This invention relates to the manufacture of refractory coatings and in particular, shell molds for use in directional solidification and for casting alloys containing reactive components.
The predominant process for making small and intricate castings such as turbine blades, vanes, nozzles and many other parts is the ceramic shell mold process. A group of expendable patterns of parts to be cast is made, for example, in wax, and set up into a cluster. This cluster is then dipped into a ceramic slurry, removed and coarse refractory is sprinkled on the wet slurry coating and allowed to harden or "set." This process is repeated several times until a sufficient thickness of ceramic is built up onto the wax pattern. Drying or chemical setting can be carried out on each layer. After the final thickness is reached, the entire assembly is "set" or dried. The wax is then removed by one of several acceptable techniques, such as in a steam autoclave or by actually firing the mold to melt out the wax. The mold is then preheated to an appropriate temperture and the metal is poured into the resulting mold.
Instead of wax, the expendable pattern may be formed of polystyrene, plastic modified wax, etc.
The usual refractories used in this system are fused silica, crystalline silica, aluminosilicates, zircon, and alumina.
Heretofore, bonding of these refractory particles has been mostly carried out by an alcoholic solution of hydrolyzed ethyl silicate or a colloidal dispersion of silica in water. Upon drying of the shell molds, the silica serves as a bond for the refractory particles. Typical ceramic shell mold processes are given in the following U.S. Pat. Nos. 3,165,799, 3,933,190, 3,005,244 and 3,955,616.
The deficiencies of silica-bonded shell molds are particularly apparent in the directional solidification technique of casting.
Such technique has been developed for producing castings having directionally solidified grains, which is particularly applicable to the manufacture of turbine blades wherein the blade has longitudinal grains, whereby the high temperature properties are improved as a result of the grain structure. One of the techniques used in producing such structures is described in the Ver Snyder U.S. Pat. No. 3,260,505. Because of the long slow cooling rates, the alloys poured, which many times contain some relatively reactive constituents, are left exposed to the hot mold for long periods of time. With silica bonds, such exposure causes a reaction with the bond by some alloys and produces a casting having a relatively poor surface and relatively poor high temperature properties.
Further when an alloy is poured into a ceramic mold, which is usually around 1800.degree. F. in normal casting operations, the alloy almost immediately solidifies, or else it solidifies immediately adjacent to the mold, because of the wide discrepancy in temperature. This solidification means a crystal formation and accordingly the casting comes out as an equiaxed grain casting. In directional solidification, the technique is to start the crystal growth from the base of a blade; for example, to grow vertically or longitudinally to form a long crystal in the direction of the blade length for best results. The less the discrepancy between the metal temperature and the mold temperature, the greater are the probabilities of being able to do this. Ideally, a mold should be at at least the solidification point of the alloy or above, so that when the metal is poured in, it will not immediately solidify adjacent to the mold surface, but then the cooling can be controlled from any direction that it is desired to do so. Therefore, by having molds that can withstand higher than normal casting temperatures, more control on grain structure can be obtained. The general maximum service temperature for conventional molds is now about 2500.degree. F. Anything above this leads to softening of the silica bonds now normally used and aggravates reactivity problems.
One attempt to overcome the reactivity problems with silica molds is described in U.S. Pat. No. 3,933,190 relating to the use of an aluminum polyoxychloride binder with an alumina refractory to form the mold. However, this type of binder has very poor green and elevated temperature strengths, thereby making it difficult to dewax the mold without cracking and destroying the mold surface. Likewise the aluminum polyoxychloride is soluble in steam, which does not permit the mold to be autoclave dewaxed.
Some observers have shown that alumina is relatively inert compared to silica with most nickel and cobalt based alloys containing minor quantities of reactive components and thus a satisfactory all-alumina shell is highly desirable.
A satisfactory all-alumina shell mold is described in Ser. No. 889,142 of the present inventor, filed Mar. 20, 1978, however, it employs a fibrous type colloidal alumina which is a rather expensive component.